US3776628A - Photoelectrophoretic imaging system - Google Patents

Abstract

A photoelectrophoretic imaging system is disclosed wherein a thermoconductive material such as polyethylene is used as the blocking electrode of the system. The thermoconductor is heated to discharge static electricity accumulated on its surface and cooled to restore its high insulating property. The heating and cooling is accomplished by using heated rollers, gas streams and infrared radiation.

Description

United States Patent 1191 Gundlach 1 Dec. 4, 1973 PHOTOELECTROPHORETIC IMAGING 3,535,221 10/1970 Tulagin 96/13 x SYSTEM 3,676,313 7/1972 Ciccarelli.

. 3,595,771 7/1971 Weigl 96/12 X [75] Inventor: Robert W. Gundlach, Victor, NY.

[73] Assigneez Xerox Corporation, Rochester, NY. Primary Examiner Roben P Greiner [22] Filed: Oct. 15, 1971 AttorneyJames J. Ralabate et a1.

[21] Appl. No.: 189,729

Related US. Application Data [62] Division of Ser. No. 829,624, June 2, 1969, Pat. No.

[52] US. Cl 355/3, 96/13, 355/4 [51] Int. Cl G03g 15/00 [58] Field of Search 355/3, 4, l7;

[56] References Cited UNITED STATES PATENTS 3,586,615 6/1971 Carreira 96/].3 X

cooling is accomplished by using heated rollers, gas

streams and infrared'radiation.

1 Claim, 4 Drawing Figures 1 PHOTOELECTROPHORETIC IMAGING SYSTEM This is a division, of application, Ser. No. 829,624, filed June 2, 1969 now US. Pat. No. 3,639,224.

BACKGROUND OF THE INVENTION This invention relates in general to imaging systems. More specifically, the invention concerns an improved photoelectrophoretic imaging system.

There has beenrecently developed an elecrophoretic imaging system capable of producing color images which utilizes electrically photosensitive particles. This process is described in detail and claimed in US. Pat. Nos. 3,384,566 to H. E. Clark, 3,384,565 to V. Tulagin et al., and 3,383,993 to Shu-Hsiung Yeh. In such an imaging system, variously colored light absorbing particles are suspended in a non-conducting liquid carrier. The suspension is placed between electrodes, one of which is generally conductive, called the injecting electrode and the other of which is generally insulating and called the blocking electrode. One of these electrodes is at least partially transparent to activating electromagnetic radiation. The suspension is subjected to a potential difference between the electrodes across the suspension and exposed to an image through said transparent electrode. As these steps are completed, selective particle migration takes place in image configuration, providing a visible image at one or both of the electrodes. An essential component of the system is the suspended particles which must be electrically photosensitive and which apparently undergo a net change in charge polarity upon exposure to activating electromagnetic radiation when within interaction range of one of the electrodes. In a monochromatic system, particles of a single color are used, producing a single colored image equivalent to conventional black-and-white photography. in a polychromatic system, the images are produced in natural color because mixtures of particles of two or more different colors which are each sensitive to light of a specific wavelength or narrow range of wavelengths are used.

This system, using a conductive injecting electrode, a substantially insulating blocking electrode and photosensitive particles dispersed in an insulating carrier liquid between the electrodes has been found to be capable of producing excellent images The insulatimg properties of the blocking electrode have been found to affect the quality of images produced by the system. Materials having a volume resistivity in the order of l to ohm-centimeters (cms) have produced satisfactory images but improved image quality is seen when the resistivity of the blocking electrode is in the order of 1O ohm-ems or greater. However, the materials having the 10 ohm-cms resistivity have not proved successful in cycling imaging situations because of the accumulation of undesirable charge on their surfaces. The reason for this may be understood by a consideration of the time constant 1 in seconds for the discharge of static electricity from an insulator having a dielectric constant K and resistivity p in ohm-ems, which may be calculated to be 1-=8.85 X 10 K p where the numerical constant has the units seconds ems/ohm. Practical blocking electrodes have dielectric constants between 2 and 200 or more, typically about 10. For a resistivity of p-lO ohm-ems therefore the time constant for 63 percent discharge of accumulated static is 0.885 K, or about 1.8 to 180 seconds, typically about 9 seconds.

For p-lO ohm-ems these times per ten times longer. Since commercial machines must frequently cycle every 0.1 to I second, it is necessary to discharge the blocking layer at least this rapidly between cycles. It is necessary to provide electrode layers which are blocking, and therefore insulating, at the imaging station, but which may be rendered temporarily sufficiently conductive, with, e.g., p less than about 10 ohm-cms and preferably 10" l0 ohm-cms to allow rapid discharge between cycles.

One solution to the above problem is to-use a photoconductive material at the blocking electrode. Following the imaging step the photoconductor is separated from the imaging suspension and flooded with light to render it conductive and thereby bleed the charge from its surface. This technique, although highly successful, has an inherent limitation in that the photoconductor is also subjected to light during the imaging step. This limitation is not always severe because the imaging suspension shields the photoconductor at least partially during the imaging step. Also, photoconductive materials are available which are either not sensitive to the imaging light or are much less sensitive to the imaging light than the imaging suspension. Nonetheless, it is desirable to devise a highly insulating blocking electrode that can be rapidly discharged in response to interaction with a medium other than light because the imaging suspension is also sensitive to light. Practical engineering considerations suggest the restriction of the quantity of light used in the system. Furthermore, the presently known materials available for use as blocking electrodes do not all have ideal characteristics in the areas of humidity resistance, cleanability, surface smoothness, abrsion resistance and cost. Consequently, there is a continuing need for improved blocking electrode materials and for methods of improving the performance of these materials in the photoelectrophoretic process.

It is therefore an object of this invention to increse the speed at which successive images can be formed by the photoelectrophoretic imaging process. Also, it is an object of the invention to overcome the above mentioned disadvantages.

Still another object of the invention is to broaden the range of materials available for the blocking electrodes of a photoelectrophoretic imaging system by using novel means in the photoelectrophoretic system for varying the resistivity of the blocking electrode material.

A further object of the invention is to devise means for dissipating charge accumulated-on the surface of the blocking electrode without interfering with the steps of the photoelectrophoretic process.

The foregoing and other objects of the present invention are attained by employing a thermoconductive material on the blocking electrode of a photoelectrophoretic system. A thermoconductive material is a material whose resistivity varies with temperature. In the present invention the thermoconductive material is insulating, i.e. has a high resistance, at the imaging step of the photoelectrophoretic process, is subsequently removed from the imaging station and heated to reduce its resistance, thereby enabling residual charge on its surface to be rapidly discharged. Thereafter the thermoconductor is cooled to restore its high resistance and a subsequent imaging operation is initiated. The present invention, therefore, provides means for rap- 3 idly discharging static electricity accumulated on the surface of a blocking electrode by effecting heat transfers with the surface of the electrode. In addition, because the present invention is not dependent upon light for its operation, it has the distinct advantage over a system using a photoconductor blocking electrode in that it eliminates the possibility of 'a discharging light source interfering with the imaging operation and of the imaging light source interfering with the blocking electrode operation.

Any suitable temperature sensitive material may be used as the thermoconductive blocking electrode. It may be homogeneous or a mixture of two or more materials and may be organic or inorganic. In addition, it may comprise heat sensitive particles dispersed in a heat resistant binder. The presently preferred material for use as the thermoconductive blocking electrode is polyethylene, since it has the dielectric strength to withstand the voltages employed in the photoelectrophoretic process and because it has a sharp decrease in resistance at temperatures to which it is readily heated and from which it is readily cooled.

DESCRIPTION OF THE DRAWINGS The advantages of this improved method of imaging will become apparent upon consideration of the detailed disclosure of the invention especially when taken in conjunction with the accompanying drawings wherein:

FIG. 1 is a schematic side-sectional view of a simple photoelectrophoretic imaging system utilizing a thermoconductive blocking electrode with conduction rollers in contact therewith for heating and cooling the thermoconductor;

FIG. 2 is a partial view of the system in FIG. 1 illustrating another embodiment of the present invention wherein the heating and cooling of the thermoconductive blocking electrode is accomplished by directing streams of gas onto the blocking electrode;

FIG. 3 is a partial view of the system in FIG. 1 illustrating another embodiment of the invention wherein an infrared radiation source is used to heat the surface of the thermoconductive blocking electrode; and

FIG. 4 is a graph of resistance vs. temperature for polyethylene.

DESCRIPTION OF THE DRAWINGS Referring now to FIG. I there is seen a transparent electrode generally designated 1 which in this exemplary instance is made up of a layer of optically transparent glass 2 overcoated with a thin optically transparent layer 3 of tin oxide commercially available under the name NESA glass. This electrode will hereafter be referred to as the injecting electrode. Coated on the surface of injecting electrode 1 is a thin layer 4 of finely divided electrically photosensitive particles dispersed in an insulating liquid carrier. Electrically photosensitive for the purposes of this disclosure refers to the properties of a particle which, once attracted near the injecting electrode, will migrate away from it under the influence of an applied electric field when it is exposed to actinic electromagnetic radiation. For a detailed theoretical explanation of the apparent mechanism of operation of the imaging process, see US. Pat. No. 3,384,565 issued May 21, 1968 to V. Tulagin et al. and US. Pat. No. 3,384,566 issued May 21, 1968 to H. E.

Clark, the disclosures of which are incorporated herein by reference.

Adjacent to liquid suspension 4 is a second electrode 5, hereinafter called the blocking electrode which is connected to one side of potential source 6 through switch 7. The opposite side of potential source 6 is connected to the injecting electrode 1 so that when switch 7 is closed an electric field is applied across the liquid suspension 4 between electrodes 1 and 5. An image projector made up of a light source 8, a transparency 9, and a lens 10 is provided to expose the dispersion 4 to a light image of the original transparency 9 to be reproduced. Electrode 5 is made in the form of a roller having a conductive central core or substrate 11 connected to potential source 6. Core 11 is covered with a layer of a thermoconductive insulating material 12 which may be any suitable thermoconductive insulating material as discussed further below. The particle suspension is exposed to the image to be reproduced while a potential is applied across electrodes 1 and 5 by closing switch 7. Roller 5 is caused to roll across the top surface of injecting electrode 1 with switch 7 closed during the period of image exposure. This light exposure causes exposed pigment particles originally attracted to electrode 1 to migrate through the liquid and adhere to the surface of blocking electrode 5, leaving behind a particulate image on the injecting electrode surface which is a duplicate of the original transparency 9. After exposure, the relatively volatile carrier liquid evaporates off leaving behind the particulate image. This particulate image may then be fixed in place, as for example, by placing a lamination over its top surface or by virtue of a dissolved binder material in the carrier liquid such as paraffin wax or other suitable binder that comes out of the solution as the carrier liquid evaporates. In an alternative, the particulate image remaining on the injecting electrode may be transferred to another surface and fixed thereon. This system can produce either monochromatic or polychromatic images depending upon the type and number of pigments suspended in the carrier liquid and the color of light to which the suspension is exposed in the process.

Where the above imaging steps are repeated, with cleaning of the blocking electrode and reapplication of the particle suspension onto the injecting electrode but without discharging the blocking electrode between imaging operations, it has been found that there is a steady decrease in the image quality in successive copies. It has been found that this gradual decrease in image quality is due to the accumulation of undesired electrostatic charge on the surface of the blocking electrode. Therefore in accordance with this invention, before or after the blocking electrode has been cleaned of unwanted particles, thermoconductive layer 12 is heated to lower its resistivity and thereby to rapidly discharge accumulated charge through layer 12 to central core 11.

The roller blocking electrode configuration shown in the drawings are of course merely representative and any other suitable configuration may be used. Typically, the blocking electrode is comprised of a thermoconductive layer mounted on a conductive substrate and may be in the form of a moveable or stationary flat plate, or in the form of a belt entrained over rollers.

The presently preferred material for the thermoconductive layer 12 is medium density polyethylene which has an electrical volume resistivity in the order of ohm-ems at 23 C and 50 percent relative humidity and a dielectric constant in the order of 2.3. The decay time, T 0.885 PK 10*, required to dissipate charge on its surface is therefore in the order of 1,840 seconds. Now in accordance with the present invention, lowerting the resistivity of the polyethylene down to 10 or l0 ohm-ems or lower reduces the decay time T to generally 0.l8 to 1.8 seconds which is well within the cycling period of a high speed imaging system.

The curve in FIG. 4 is a plot of volume resistivity vs. temperature for polyethylene coated paper. The curve illustrates that a significant change in the resistivity of polyethylene is obtainable by heating and cooling the material. Consequently, the heating and cooling of the blocking electrode thermoconductor enables the best of two situations to be obtained. Heating the thermoconductive materials lowers its resistivity thereby enabling surface static electricity to be rapidly discharged. Cooling the thermoconductor restores its high resistance permitting optimum performance during the imaging operation. The specific temperature between which the thermoconductive material is raised and lowered depends upon, among other parameters, the particular thermoconductive material used and the duration of the cycle time at which the imaging system operates. Obviously, the preferred temperatures are selected for optimum performance for a given imaging system.

Generally, most materials experience a variation in resistivity with variations in temperature. Amongst these class of materials are photoconductors and as such they may be employed in the manner prescribed by the present invention. A material available from the DuPont Corporation under the name Tedlar PVF film is another example of a material having electrical and thermal characteristics suitable for use as a blocking electrode in acccordance with the present invention. TedlarPVF film has a volume resistivity of 3 X l0 ohm-cms at 23 C and of 1 X l0 ohm-cms at 132 C. Some materials experience an increase in resistivity with an increase in temperature which is the inverse of what happens to polyethylene. If these materials have the desired dielectric constants and resistivities they can be used in accordance with the present invention. In this case however, the material would be heated to render it sufficiently resistive for the imaging step and cooled to bleed the charge from its surface.

The thermoconductive layer 12 shown in FIGS. 1, 2, and 3 is relatively thin (approximately one thirtysecond inch) and therefore is a mass which is rapidly heated and cooledwithin a short period of time. Consequently, variations in the resistivity of the blocking electrode can be made within a time period consistent with the cycle period of a high speed imaging system.

In FIG. 1, the heating and cooling of the thermoconductive layer 12 is accomplished by the conduction rollers 16 and 17. The peripheries of the conduction rollers are in contact with the thermoconductor 12 to heat and cool the surface by conduction through the rollers. The hot roller 16 has an outer layer 18 of steel (or other material that is a good conductor of heat) and a heating element 19 within the core of layer 18. The heating element 19 includes an electrical heating coil but can be any other suitable heating element such as those using hot gasses or liquids. The cold roller 17 also has an outer layer 21 of steel (or other material that is a good conductor of heat) and a refrigeration element 22 within the core of layer 21. The refrigeration element has coiled conduits through which cooled gases or liquids are circulated in order to lower the temperature of the cold roller. Because the outer layer 18 of the hot roller 16 is also electrically conductive, it is coupled to a ground potential by means of the conductive lead 23. Some charge therefore may be bled from the surface of the thermoconductive layer 12 by virtue of the contact between the roller 16 and the thermoconductor.

In FIG. 2, the heating and cooling of thermoconductive layer 12 is accomplished by means of the nozzles 25 and 26 which direct streams of hot and cold gas onto the surface of the thermoconductive layer 12. The hot nozzle 25 is coupled to a source of pressurized gas 27, such as compressed air, by the coiled conduit 28. The air in conduit 28 is heated by the electrical heating element 29 positioned within the coils of conduit 28. Similarly, the cold nozzle 26 is coupled by coiled conduit 31 to the gas source 27. The air in conduit 31 is cooled by the refrigeration element 32 positioned within the coils of conduit 31. The hot and cold gas streams emitted by the nozzles therefore heat and cool the surface of the thermoconductor by conduction and convection. Each of the noules has a diaphragm adjacent the exit orifice to permit the air stream to be turned on and off.

In FIG. 3, the thermoconductor is again cooled by the cold nozzle 26 but in this case is heated by the infrared lamp 33. The rays of lamp 33 are focused to a narrow area across the width of thermoconductive layer by means of a condensing lens 34.

The power units for driving the various heating and cooling elements such as the electrical heating coils 19, 29 and 31, the infrared lamp 33 and the cooling or refrigeration coils 22 and 32 are of conventional design and are not illustrated in the figures in order to simplify and thereby clarify the description.

In each of the above embodiments of the present invention, the majority of the heat transfer with the thermoconductive layer 12 takes place over a narrow area across its width. The remaining surface area of the thermoconductor is subjected to the heat transfers by tuming the roller through 360. Proceeding in this manner permits relatively small amounts of energy to be used for effecting the heat transfers by minimizing the thermal mass involved in the thermal cycling and permits the heat transfers to occur within periods of time consistent with the operation cycle of a high speed imaging system. Furthermore, controlling the thickness of the thermo-conductive layer and limiting the area over which the layer is heated increases the effectiveness of the apparatus used to cool the thermoconductive layer and can eliminate the need for such apparatus as cold roller 16 and cold nozzle 26 in certain system configurations. The reason is that the heat transfer to the thermoconductor is over a relatively'small portion of its total surface area and accordingly is capable of being rapidly dissipated by radiation and conduction to the air and to the core of the roller.

The specific quantities of heat required to be transferred to and from the thermoconductor depends upon the characteristics of a selected thermoconductive material and the system in which it is used. Enough heat must be transferred to the thermoconductive layer to raise the temperature to a level at which charge is rapidly dissipated. Likewise, enough heat must be transferred from the therrnoconductor or lower the temperature to a level at which it is highly insulating. The specific temperatures between which various materials must be varied to obtain the desired results will depend on the values of resistivity as a function of temperature, and the area over which it is heated and cooled at any point in time.

What is claimed is:

l. A photoelectrophoretic imaging apparatus comprising a first electrode including a thennoconductive insulating layer, a second electrode, means to bring said first electrode into and out of virtual contact with said second electrode,

means to impose an electric field between said electrodes,

exposure means for exposing a photoelectrophoretic imaging suspension applied between said electrodes to imagewise activating electromagnetic radiation to form an image on at least one of said electrodes from said suspension exposed to radiation and subjected to an electric field established between the electrodes, and

means to effect a heat transfer with said thermoconductive layer to lower its resistivity for dissipating accumulated charge thereon to prepare said apparatus for reuse.

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